A typical powertrain includes an engine and several pumps, including an engine oil pump, a cooling fan, a transmission pump, a coolant pump, and various compressors. The rotors of the pumps are typically driven by the engine crankshaft, and therefore the rotational speed of the rotors, and the power delivered to the pumps, are dependent on the speed of the crankshaft. However, the speed of the crankshaft is dictated by the requirements of a primary power consuming device, such as a vehicle drivetrain or electrical generator, and not the requirements of the pumps.
Accordingly, some pumps must be sized to achieve maximum required pressure or fluid flow at low crankshaft speeds; therefore, the pumps may produce more pressure or fluid flow than is actually required by the powertrain when the crankshaft rotates at higher speeds. When producing more pressure or fluid flow than is actually necessary or desired, the pumps use more power from the crankshaft than is actually necessary, thereby reducing the efficiency of the powertrain.
For example, maximum required oil flow occurs when an engine operates at peak torque output. Peak torque output may occur at a crankshaft speed that is less than a typical operating crankshaft speed range. Thus, the oil pump must be sized to achieve the maximum required oil flow at a crankshaft speed that is lower than the typical engine operating speed range; when the engine is operated within the typical operating crankshaft speed range, the oil pump produces more oil flow than is required, and a pressure bypass valve diverts excess pump flow, resulting in unnecessary pump power usage and parasitic energy loss from the powertrain.
Similarly, the amount of fluid flow required to be produced by a pump, and accordingly the amount of power required by the pump, may vary significantly with various powertrain operating parameters and conditions. However, because the speed of the pump rotor, and accordingly the power used by the pump, is controlled by the speed of the crankshaft, the pump must be sized to produce the maximum flow rate that may be required at any given crankshaft speed.
For example, an engine cooling fan is typically driven by the crankshaft. Although the amount of air flow required by the powertrain may vary significantly with vehicle speed, ambient atmospheric temperature, etc., the fan must be sized to produce the maximum amount of air flow that may be required at any given engine speed. Accordingly, the fan may generate more air flow than conditions require, and therefore may use more power from the crankshaft than conditions require.
Similarly, a transmission pump is typically driven by a crankshaft and provides pressurized fluid to lubricate and cool the transmission parts, and to actuate torque transmitting devices such as clutches and brakes to effectuate speed ratio changes. However, the amount of fluid flow and pressure to the transmission may vary depending on speed ratio shift activity, engine speed, engine load, etc. Accordingly, the transmission pump may generate more fluid flow and pressure than conditions require; excess fluid from the pump is typically exhausted to a reservoir.
Various prior art mechanisms attempt to overcome the shortcomings inherent in having pump speed directly determined by crankshaft speed. In hybrid vehicles driven by an engine and a motor, a mode of operation is possible in which the engine is off and the vehicle is driven solely by the motor. The prior art includes powertrains with two pumps, one being driven by the crankshaft of the engine, and another being driven by the rotor of the motor when the engine is off. However, having two pumps results in additional mass, cost, and mechanical complexity. Moreover, the motor in such hybrid vehicles also drives the vehicle, and, therefore, the speed of the motor-driven pump may be dictated by the requirements of the vehicle drivetrain and not by the pump, which results in the same inefficiencies noted above pertaining to crankshaft-driven pumps.
Some prior art powertrains, such as the one disclosed by Moses et al. in U.S. Pat. No. 6,964,631, include a motor that drives a pump via a freewheel clutch only when the speed of a motor-driven element exceeds the speed of a crankshaft-driven element. Accordingly, when the engine is off, the motor can drive the pump.
The prior art also includes pump systems, such as the one disclosed by Kopko in U.S. Pat. No. 5,947,854, in which a primary motor and an auxiliary motor are connected to a pump via an epicyclic gearing system. The primary motor is operated at a constant speed, and the auxiliary motor is driven at variable speeds to control the speed of the pump rotor. However, the speed of the primary motor is constant, and both the primary and auxiliary motors drive only the pump. Accordingly, the pump system of Kopko is not applicable to typical powertrains in which the speed of the engine is variable and is dictated not by the pump, but by another power consuming device.
In U.S. Pat. No. 2,505,713, Lucia discloses a supercharger compressor that is connected to an engine crankshaft via epicyclic gearing to be driven thereby. An exhaust driven turbine is selectively connectable to the epicyclic gearing via a freewheel clutch when the turbine speed exceeds the speed of a crankshaft-driven member. However, the speed of the turbine is dependent upon the speed of the crankshaft (the amount of exhaust driving the turbine is related to engine speed and engine load), and therefore the turbine's efficacy in providing power to the supercharger is directly related to crankshaft speed.
Dougan et al. disclose, in U.S. Pat. No. 6,695,589 a transmission pump that is driven by an electric motor. However, the motor must be of sufficient size to power the pump by itself, which may increase the mass of the powertrain and require additional packaging space. Moreover, driving the pump solely by the electric motor introduces inefficiencies since, assuming that the power source for the electric motor, such as a battery, is charged by the engine, energy losses are incurred when rotary power from the crankshaft is converted to chemical energy in the battery, and when the chemical energy is converted to electrical energy for the motor, and again when the electrical energy is converted to rotary power in the motor.
The present disclosure is directed to one or more of the problems set forth above.